Disturbance in utility and mains supply voltages can be a major problem for industrial and commercial users that depend on electronic equipment for factory and office automation. Voltage sags have been identified as being one of the most commonly occurring disturbances. Mains voltage sags of sufficient magnitude can cause electrical or electronic equipment to malfunction or shut down, which can be very costly, especially in continuous process applications. One known solution to this problem is to install onsite active voltage conditioning units that are arranged to detect voltage disturbances or sags in the supply and inject a corrective voltage into the supply to compensate for the voltage disturbances or sags and ensure reliability.
A known active voltage conditioner configuration is shown in FIG. 1. The active voltage conditioner 1 is connected to the output of a local distribution transformer 3 that distributes a three-phase mains supply 5. The active voltage conditioner 1 comprises a three-phase voltage source inverter 7, a bypass circuit 9, and an injection transformer 8 connected in a series to each other between the incoming main supply from the distribution transformer 3 and a load 4 connected to the active voltage conditioner 1. A control system of the active voltage conditioner 1 monitors the incoming supply voltage and when it deviates from the nominal voltage level, the control system inserts an appropriate compensating voltage using the inverter 7 and series injection transformer 8 to regulate the load voltage 4 to the nominal value, in an attempt to eliminate voltage disturbances from the mains supply affecting the load. Energy (potential) for the compensating voltage is provided from a three-phase rectifier, which is connected to the input supply, directly or via a transformer or autotransformer, and which can supply or remove power from the inverter 7 as required. The rectifier controls power flow in and out of the input supply from the distribution transformer 3 in such a way as to hold the inverter 7 input direct current (DC) bus supply at a constant value. In this way, the control system automatically acts to provide energy balance drawing extra power from the input supply when required or supplying it back to the input supply if the voltage correction results in excess regenerated energy.
The control system typically utilizes a digital signal processor (DSP) microprocessor-based system that is arranged to calculate any vectorial voltage differences from a perfect balanced and regulated three-phase supply, and then use these differences to calculate and create appropriate pulse width modulated (PWM) waveforms to control the inverter 7 to insert an appropriate compensating voltage in both phase and magnitude on individual phases via the series injection transformer 8. For instance, the DSP is arranged to sample the incoming mains supply and calculate the correction or compensation voltage to be applied through the injection transformer 8 to restore the output to a regulated, balanced three phase sinusoidal supply, or as close as possible to this within the correction capabilities of the control system. The three phase voltages of the mains supply are measured in real time and then transformed into the stationary reference frame where they are represented as alpha and beta terms (values). This is an application of standard vector control principles that are known in the art. The DSP then calculates the alpha and beta voltage compensation terms, Va and Vb, necessary to bring the utility supply back to the set nominal level. The DSP then utilizes Va and Vb to generate the PWM waveforms for controlling the inverter 7 to generate and apply the appropriate compensation voltage(s) to the primary terminals of the injection transformer 8.
Various similar active voltage conditioning configurations are proposed and described in U.S. Pat. Nos. 5,319,534, 5,610,501 and 6,327,162, which are incorporated herein by reference. All these configurations also utilize the inverter fed injection transformer topology for regulating the supply voltage to a load.
During operation of such active voltage conditioners, when step or sudden changes are made to the voltage of the primary of the injection transformer, the transformer core flux adjusts in proportion to the applied voltage and there is also normally a flux offset present. Subsequent changes in voltage will add and subtract from this flux offset and this can make the peak core flux levels larger or smaller depending on the phase and magnitude of the voltage changes. Therefore, there is a concern for the occurrence of core magnetic saturation, which results in very high inverter currents and possible loss of system control. This attribute can reduce the reliability and effectiveness of the active voltage conditioner.
In this specification, where reference has been made to external patent specifications, other external documents, or other sources of information, such reference is generally for the purpose of providing a context for discussing various features of the present disclosure. Unless specifically stated otherwise, reference to such external documents is not to be construed as an admission that such documents, or such sources of information, in any jurisdiction, constitute prior art, or form part of the common general knowledge in the art.
Exemplary embodiments of the present disclosure provide a flux control system for reducing the risk of core magnetic saturation in an injection transformer of an active voltage conditioner.